How long do I have to be married before I can file as married filing jointly?

So long as you are officially married on the last day of the taxable year, you can file your taxes jointly. However, this is a double-edge sword – even if you are married the entire year, but are officially divorced on the last day, then you cannot file your tax return jointly and gain the benefits of joint returns. However, being separated (and not officially divorced) does not allow you to file a single return. Unless your divorce is complete, then you must either file jointly or married filing separately.